EP3517998B1 - Airborne lidar pulse rate modulation - Google Patents
Airborne lidar pulse rate modulation Download PDFInfo
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- EP3517998B1 EP3517998B1 EP18153160.9A EP18153160A EP3517998B1 EP 3517998 B1 EP3517998 B1 EP 3517998B1 EP 18153160 A EP18153160 A EP 18153160A EP 3517998 B1 EP3517998 B1 EP 3517998B1
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- laser scanner
- optical element
- laser pulses
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- 230000010355 oscillation Effects 0.000 claims description 12
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/481—Constructional features, e.g. arrangements of optical elements
- G01S7/4817—Constructional features, e.g. arrangements of optical elements relating to scanning
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/483—Details of pulse systems
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/06—Systems determining position data of a target
- G01S17/08—Systems determining position data of a target for measuring distance only
- G01S17/10—Systems determining position data of a target for measuring distance only using transmission of interrupted, pulse-modulated waves
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/89—Lidar systems specially adapted for specific applications for mapping or imaging
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/93—Lidar systems specially adapted for specific applications for anti-collision purposes
- G01S17/933—Lidar systems specially adapted for specific applications for anti-collision purposes of aircraft or spacecraft
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/483—Details of pulse systems
- G01S7/484—Transmitters
Definitions
- the present invention relates to an airborne laser scanner according to the generic term of claim 1.
- Scanning a target, such as the ground surface, with a laser range finder for creating 3D point clouds and derivative products is often achieved by moving, in particular rotating or oscillating an optical element such as a mirror, wedge or prism.
- an optical element such as a mirror, wedge or prism.
- short laser pulses are transmitted and directed towards a target surface according to a defined scan pattern using movable mirrors or refractive optics.
- the distance to the ground, i.e. the target surface can be significant (up to 5 km) and the scanning rate of the scan pattern (typical 200-300 rad/s) can be quite high.
- This type of scanning usually results in a point distribution that varies over the swath of the scan.
- the uneven point density may affect the quality of the end product.
- For an oscillating or rotating scan there will be a significant pile-up of points at the edges of the swath.
- the present invention aims at providing an improved airborne laser scanner that allows surveying a target with a more even measuring point distribution. This aim is achieved by modulating the pulse rate of the laser in such a way that the variation of density, as well as the pile-up effect on the edges are minimised.
- the laser of the airborne scanner is configured to emit pulses on demand, in particular with nearly constant pulse energy causing the amplitude of the emitted pulses to be independent of the repetition rate.
- Generic fibre lasers usually have a pulse energy that can vary significantly with change in pulse repetition rate.
- Fibre lasers known from prior art are usually configured to only regulate the average emitted optical power by the current driven into the pump diodes.
- the pulse energy is proportional to the average power divided by the pulse rate. So if the pulse rate changes while the average power is constant, the pulse energy increases with decreasing pulse rate and vice versa.
- the laser In order to emit pulses on demand with a constant energy the laser has to track the applied pulse rate and adjust the average power accordingly in real time. This way, the average optical power varies as a function of the applied pulse rate but the energy contained in each emitted laser pulse is kept at a constant level.
- the principle of the invention is to generate the trigger signal for the laser depending at least on the position, in particular and the orientation, of the scanning device.
- a trigger pattern By applying such a trigger pattern one can achieve a larger temporal spacing of the pulses at the edges of the swath which causes a point density similar to the central part of the swath. This results in a more even spatial distribution of the laser pulses.
- pulses for consecutive scans will line up better and give a cleaner and more homogeneous point distribution.
- the lower pulse rate at the edges of the swath reduces the number of pulses that end up at the same geographical location, thus the eye-safety is improved. This allows for a less harmful operation of the scanning device for observers on the ground and for a higher peak power of the laser itself.
- the laser is completely disabled during the edge region of the scanning area to reduce the number of pulses piling up to zero there. As the scan is moving more or less in the direction of the flight, the effective swath width will only slightly be reduced.
- the scanner position may be not available in real time, or not available at a desired rate.
- the trigger events may then be generated as a time series (not evenly distributed) that is kept synchronised with the scanner movements. At least the eye safety issue can be handled with similar performance as if using the scanner position directly.
- the point distribution performance is generally more critical since it depends on placing each individual shot accurately in relation to its surrounding. This embodiment would require very strict time synchronisation.
- the invention solves the problem of a nonuniform point distribution on the target surface, improves the eye-safety conditions, allows for higher laser power and thus increases both the signal to noise ratio (SNR) and overall sensitivity of the scanning device.
- SNR signal to noise ratio
- the invention relates to an airborne laser scanner configured to be arranged on an aircraft for surveying a ground surface along a flight path of the aircraft, wherein the airborne laser scanner comprises an emitter configured for emitting a plurality of consecutive laser pulses towards the ground surface, at least one optical element configured for deflecting the laser pulses along pulse paths towards the ground surface, a motor configured for altering the pulse paths by moving the optical element, a receiver configured for receiving the laser pulses backscattered from the ground surface, and a computer configured for controlling the emitter, the motor, and the receiver, collecting time stamps of the emitted laser pulses and the received laser pulses or collecting distance values calculated based on the time stamps of the emitted laser pulses and the received laser pulses, and determining directions of the pulse paths, wherein surveying the ground surface is for generating a three-dimensional point cloud based on the time stamps and on the directions of the pulse paths, wherein the computer is configured for triggering the emitter to emit the laser pulses with a varying pulse spacing based on
- the computer may be configured for determining distances from the airborne laser scanner to the target based on emitted and received laser pulses.
- the determining of distances from the airborne laser scanner to the ground surface may be based on the time of flight of the emitted and received laser pulses.
- the computer may further be configured for generating a three-dimensional point cloud based on the distances and the directions.
- the flight path may be considered a direction of travel of the laser scanner.
- the pulse spacing may be gradually varied between a minimum pulse spacing and a maximum pulse spacing, wherein the minimum pulse spacing is set when the directional component is minimal, and the maximum pulse spacing is set when the directional component is maximal.
- the pulse spacing may be gradually varied according to a sinusoidal characteristic, a linear zig-zag characteristic, a wave characteristic, a saw tooth characteristic, or a step characteristic.
- the characteristic of the pulse spacing can have a characteristic being any combination of said optional characteristics.
- the optical element is a prism.
- the optical element is a mirror.
- a prism may be any type of prism, for example a wedge prism.
- the motor is configured for rotating the optical element around a rotation axis, resulting in a cone-shaped laser pulse emission pattern.
- This rotation may be performed continuously, i.e. infinite, or the rotation direction may reverse at a reverse position resulting in a half-cone-shaped pulse emission pattern.
- the airborne laser scanner comprises an angle encoder configured for providing positions of the optical element.
- Said positions of the optical element are in particular rotational positions or angular positions respectively.
- the directional components of the pulse paths in a horizontal direction perpendicular to a direction of the flight path can be derived from the positions of the optical element.
- the motor may be configured for oscillating the optical element around an oscillation axis, resulting in a fan-shaped laser pulse emission pattern.
- the airborne laser scanner according to a comparative example comprises an oscillation sensor configured for providing positions of the optical element.
- the computer may be configured for calculating a time-based sequence of the varying pulse spacings. Said sequence can be applied without a real-time determination of the deflection direction. However, in this case, the applied sequence should be synchronised with the rotation or oscillation of the optical element.
- the optical element may be arranged relative to the emitter in such a way that the optical element deflects the laser pulses in a defined constant angle relative to the rotation axis or the oscillation axis.
- the computer may be configured for determining a current of the motor, and determining the directions of the pulse paths based on the current. In particular, also the defined constant angle is taken into account when determining the directions of the pulse paths.
- the computer may also be configured for receiving flight data from the aircraft, wherein the flight data may comprise a direction of the flight path of the aircraft.
- the flight data are time-based GNSS-signals.
- One embodiment of the airborne laser scanner comprises an Inertial Measuring Unit (IMU), wherein the computer may be configured for receiving heading data from the IMU and determining a direction of the flight path based on the heading data.
- IMU Inertial Measuring Unit
- the computer and the emitter may be configured for providing the laser pulses with constant pulse energy.
- Constant pulse energy is to be understood as at least essentially the same pulse energy, e.g. granting a 10% tolerance for such restriction.
- the optical element may be configured for deflecting the laser pulses backscattered from the target towards the receiver.
- Figure 1 shows an aircraft 1 flying over a ground surface as target 3, a flight path 6 (trajectory) of the aircraft 1, an airborne laser scanner 2 according to the invention arranged on the aircraft 1, laser pulses 4 emitted by the airborne laser scanner 1, and a point cloud 5 of the swath of the aircraft 1 generated based on said laser pulses.
- the laser pulse emission pattern 4 is symbolically shown as a pyramid. In reality however it rather appears as a cone or fan.
- Figure 2a symbolically shows an embodiment of an airborne laser scanner 2 according to the invention.
- Figure 2b symbolically shows a comparative example of an airborne laser scanner 2.
- Figure 2a shows the airborne laser scanner 2 comprising a computer 26, an emitter 21 and a receiver 25, which in this example are combined in one unit, but may however also be arranged separately.
- a prism 22, in particular a wedge prism, as optical element is in operative connection with a motor 24, such that it is rotatable around the rotation axis R.
- the movement of the optical element 22 induced by the motor may be a continuous or a partial rotatory movement.
- a continuous rotatory movement is to be understood as at least one full 360°-rotation, in particular a permanent sequence of full rotations, and a partial rotatory movement is to be understood as a rotation, in particular a rotation of less than 360°, which is reversed in a defined scheme. For example, after a 180°-rotation, the motor changes the rotational direction and goes back the 180°, and so on.
- the emitter 21 emits a plurality of consecutive laser pulses 211 towards the target.
- the pulse path is altered by the movement of the optical element.
- the pulses 251 backscattered from the target are received by the receiver 25.
- the computer 26 is connected to the emitter 21, the receiver 25, and the motor 24, and it is configured for controlling these components.
- the computer 26 may additionally be configured for determining a distance from the airborne laser scanner to the ground surface for emitted and received laser pulses based on the time of flight method. Since the angle by which the optical element is deflecting the pulses and the direction of the pulse path are known and/or determinable (e.g. by an angle encoder), the TOF-distance value can be associated to the direction (e.g. at least one coordinate such as angle(s)) of the current pulse path at a specific measurement time.
- a three-dimensional point cloud based on these associations can be generated by an external computer in a post-processing.
- the internal computer 26 is merely configured to collect the data.
- the data may comprise time stamps of pulse transmission and pulse reception or distance values already calculated by means of said stamps, and transmission/reception direction.
- the computer 26 can be configured for generating said point cloud, in particular in real-time.
- the laser pulses i.e. the laser pulse rate, or the laser pulse spacings are modulated based on the directional component of the current pulse path in a horizontal direction perpendicular to a direction of the flight path.
- the invention allows generating a point cloud which has a more steady (or: even) point distribution and which has less point cluster.
- a comparative example of the airborne laser scanner 2 is shown in figure 2b , wherein the construction is similar to the one shown in figure 2a but the optical element is a mirror 23 instead of a prism.
- the motor 24 does not perform full rotations as positioning, but performs oscillations around an oscillation axis O.
- the oscillation axis runs along (or: parallel to) the surface of the mirror.
- the oscillation axis can however also run elsewhere.
- the oscillation can also be understood as a partial rotatory movement.
- the emitted laser pulses 211 are deflected towards the target and back along a pulse path. Said pulse path pivots laterally to the flight path.
- Figures 3a and 3b show the unfavourable point distribution in point clouds 5 of two exemplary laser pulse emission patterns according to prior art.
- the paper plane is the ground surface 3.
- These point clouds 5 have been generated by laser pulses which have been emitted at a constant pulse rate or constant pulse spacing. It can be recognised that the point density in the area of a reverse position 7 of the motor is relatively high compared to the area between the reverse positions 7. At this reverse position 7 the directional component of the pulse path in a horizontal direction perpendicular to a direction of the flight path is maximal.
- the pattern of figure 3a corresponds to a laser scanner of figure 2a , wherein continuous full rotations are performed while the aircraft is moving along a flight path 6.
- the pattern of figure 3b corresponds to a laser scanner of figure 2b , wherein the motor oscillates the mirror within the reverse positions 7 while the aircraft is moving along a flight path 6.
- the reverse position may in some embodiments also be defined as a position of the motor in which a laser pulse is deflected by the optical element at the largest angle with respect to a plumb-line of the aircraft and in a plane perpendicular to a flight path of the aircraft.
- an emitted laser pulse measures the fringe of the swath of the scanner, i.e. the most lateral areas with respect to the trajectory of the aircraft.
- the motor reverses the deflection to the respective other direction (towards the other edge of the swath) with respect to an axis perpendicular to the flight path 6 in figures 3a and 3b .
- Figures 4a and 4b show the result of pulse modulation.
- Figure 4a is according to the invention.
- an airborne scanner according to the invention can achieve various scan patterns.
- Figure 5a shows a double prism arrangement configured to deflect the laser path twice - once by the prism 22 and once by the prism 22'.
- Both prisms may be motorised by motor 24 and motor 24' respectively.
- said motors are configured for rotating the prisms in opposite directions.
- Figure 5b shows a double optical element arrangement configured to deflect the laser path twice - once by the mirror 23 and once by the prism 22.
- Both the mirror 23 and the prism 22 may be motorised by motors 24 and 24'.
- the oscillation of the mirror 23 may be superimposed by the rotation of the prism 22.
- Possible scan patterns resulting from the arrangements shown in figures 5a and 5b may have the shape of a flower or a spiral following a zigzag path. By varying the respective rotation/oscillation speeds and/or directions, various different patterns are achievable.
- the pulse rate is decreased the more the pulses approach the reverse lines 7, or in other words, the more the pulse paths are directed towards the edges of the swath.
- Figures 6a, 6b, 6c, and 6d each show exemplary pulse modulations provided by the computer to the emitter. The modulations all have in common that they are depending on the pulse path direction.
- the pulse spacing 8 reaches its maximum (or respectively: the pulse rate has a minimum).
- the pulse spacing 8 reaches its minimum (or respectively: the pulse rate reaches its maximum).
- the laser pulse is directed essentially onto the flight path 6 on the ground surface, i.e. said directional component of the pulse paths in a horizontal direction perpendicular to a direction of the flight path is minimal, in particular zero.
- the pulse spacing may be gradually varied according to a sinusoidal characteristic ( figure 6a ), a linear zig-zag characteristic ( figure 6b ), a wave characteristic ( figure 6c ), or a saw tooth characteristic ( figure 6d ).
- the pulse spacing is varied according to a step characteristic, wherein the pulse spacing is kept constant for a step period and then jumps up step by step.
- any combination of the above mentioned pulse sequences may be applied.
- sequences of pulses 4 indicated by the arrows in figures 6a-d are qualitative illustrations. In fact, the sequences of pulses 4 differ slightly based on the according pulse spacing characteristic 8. Said courses of pulse spacings 8 are also to be understood as qualitative illustrations, which may appear exaggerated. However, the graphs 8 are intended to show that there may be various optional mathematical principles behind the way the pulses are modulated according to the invention.
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Description
- The present invention relates to an airborne laser scanner according to the generic term of claim 1.
- Scanning a target, such as the ground surface, with a laser range finder for creating 3D point clouds and derivative products is often achieved by moving, in particular rotating or oscillating an optical element such as a mirror, wedge or prism. In the generic long range LiDAR system shown in
EP 3 264 135 A1 - Further prior art LiDARs are disclosed in
JP 2017 015416 A CN 103 076 614 B ,JP 2006 284204 A JP 2004 226133 A JP 2008 203123 A - This type of scanning usually results in a point distribution that varies over the swath of the scan. The uneven point density may affect the quality of the end product. Also, there may be a scanned region having an unnecessarily high point density resulting in waste of data memory space. For an oscillating or rotating scan there will be a significant pile-up of points at the edges of the swath.
- Many LIDAR data filter algorithms and filters depend on a certain point distribution to work optimally.
- Also the pile-up of pulses will generate a lot of laser pulses ending up overlapping each other. In terms of eye-safety this means all the overlapping pulses need to be considered when computing the maximum permissible emission of the laser system. Therefore, the present invention aims at providing an improved airborne laser scanner that allows surveying a target with a more even measuring point distribution. This aim is achieved by modulating the pulse rate of the laser in such a way that the variation of density, as well as the pile-up effect on the edges are minimised.
- In an embodiment of the invention the laser of the airborne scanner is configured to emit pulses on demand, in particular with nearly constant pulse energy causing the amplitude of the emitted pulses to be independent of the repetition rate. Generic fibre lasers usually have a pulse energy that can vary significantly with change in pulse repetition rate.
- Fibre lasers known from prior art are usually configured to only regulate the average emitted optical power by the current driven into the pump diodes. The pulse energy is proportional to the average power divided by the pulse rate. So if the pulse rate changes while the average power is constant, the pulse energy increases with decreasing pulse rate and vice versa.
- In order to emit pulses on demand with a constant energy the laser has to track the applied pulse rate and adjust the average power accordingly in real time. This way, the average optical power varies as a function of the applied pulse rate but the energy contained in each emitted laser pulse is kept at a constant level.
- The principle of the invention is to generate the trigger signal for the laser depending at least on the position, in particular and the orientation, of the scanning device. By applying such a trigger pattern one can achieve a larger temporal spacing of the pulses at the edges of the swath which causes a point density similar to the central part of the swath. This results in a more even spatial distribution of the laser pulses.
- By generating the trigger as a function of the scan position, pulses for consecutive scans will line up better and give a cleaner and more homogeneous point distribution.
- In addition, the lower pulse rate at the edges of the swath reduces the number of pulses that end up at the same geographical location, thus the eye-safety is improved. This allows for a less harmful operation of the scanning device for observers on the ground and for a higher peak power of the laser itself.
- In one embodiment, the laser is completely disabled during the edge region of the scanning area to reduce the number of pulses piling up to zero there. As the scan is moving more or less in the direction of the flight, the effective swath width will only slightly be reduced.
- In a further embodiment, the scanner position may be not available in real time, or not available at a desired rate. The trigger events may then be generated as a time series (not evenly distributed) that is kept synchronised with the scanner movements. At least the eye safety issue can be handled with similar performance as if using the scanner position directly. The point distribution performance is generally more critical since it depends on placing each individual shot accurately in relation to its surrounding. This embodiment would require very strict time synchronisation.
- Summarised, the invention solves the problem of a nonuniform point distribution on the target surface, improves the eye-safety conditions, allows for higher laser power and thus increases both the signal to noise ratio (SNR) and overall sensitivity of the scanning device.
- At least one of these improvements is achieved by the airborne laser scanner according to claim 1 and/or the dependent claims of the present invention.
- The invention relates to an airborne laser scanner configured to be arranged on an aircraft for surveying a ground surface along a flight path of the aircraft, wherein the airborne laser scanner comprises an emitter configured for emitting a plurality of consecutive laser pulses towards the ground surface, at least one optical element configured for deflecting the laser pulses along pulse paths towards the ground surface, a motor configured for altering the pulse paths by moving the optical element, a receiver configured for receiving the laser pulses backscattered from the ground surface, and a computer configured for controlling the emitter, the motor, and the receiver, collecting time stamps of the emitted laser pulses and the received laser pulses or collecting distance values calculated based on the time stamps of the emitted laser pulses and the received laser pulses, and determining directions of the pulse paths, wherein surveying the ground surface is for generating a three-dimensional point cloud based on the time stamps and on the directions of the pulse paths, wherein the computer is configured for triggering the emitter to emit the laser pulses with a varying pulse spacing based on the directional component of the pulse paths in a horizontal direction perpendicular to a direction of the flight path. In particular, the computer may be configured for determining distances from the airborne laser scanner to the target based on emitted and received laser pulses. In particular, the determining of distances from the airborne laser scanner to the ground surface may be based on the time of flight of the emitted and received laser pulses. In particular, the computer may further be configured for generating a three-dimensional point cloud based on the distances and the directions. In particular, the flight path may be considered a direction of travel of the laser scanner.
- The pulse spacing may be gradually varied between a minimum pulse spacing and a maximum pulse spacing, wherein the minimum pulse spacing is set when the directional component is minimal, and the maximum pulse spacing is set when the directional component is maximal.
- The pulse spacing may be gradually varied according to a sinusoidal characteristic, a linear zig-zag characteristic, a wave characteristic, a saw tooth characteristic, or a step characteristic. In particular, the characteristic of the pulse spacing can have a characteristic being any combination of said optional characteristics.
- In particular, increasing the pulse spacing is equivalent with decreasing a pulse rate.
- According to the invention the optical element is a prism. In a comperative example the optical element is a mirror. A prism may be any type of prism, for example a wedge prism.
- According to the invention the motor is configured for rotating the optical element around a rotation axis, resulting in a cone-shaped laser pulse emission pattern. This rotation may be performed continuously, i.e. infinite, or the rotation direction may reverse at a reverse position resulting in a half-cone-shaped pulse emission pattern.
- The airborne laser scanner according to an embodiment of the invention comprises an angle encoder configured for providing positions of the optical element. Said positions of the optical element are in particular rotational positions or angular positions respectively. In particular, the directional components of the pulse paths in a horizontal direction perpendicular to a direction of the flight path can be derived from the positions of the optical element.
- In a comparative example the The motor may be configured for oscillating the optical element around an oscillation axis, resulting in a fan-shaped laser pulse emission pattern.
- The airborne laser scanner according to a comparative example comprises an oscillation sensor configured for providing positions of the optical element.
- The computer may be configured for calculating a time-based sequence of the varying pulse spacings. Said sequence can be applied without a real-time determination of the deflection direction. However, in this case, the applied sequence should be synchronised with the rotation or oscillation of the optical element.
- The optical element may be arranged relative to the emitter in such a way that the optical element deflects the laser pulses in a defined constant angle relative to the rotation axis or the oscillation axis.
- The computer may be configured for determining a current of the motor, and determining the directions of the pulse paths based on the current. In particular, also the defined constant angle is taken into account when determining the directions of the pulse paths.
- The computer may also be configured for receiving flight data from the aircraft, wherein the flight data may comprise a direction of the flight path of the aircraft. In particular, the flight data are time-based GNSS-signals.
- One embodiment of the airborne laser scanner comprises an Inertial Measuring Unit (IMU), wherein the computer may be configured for receiving heading data from the IMU and determining a direction of the flight path based on the heading data.
- The computer and the emitter may be configured for providing the laser pulses with constant pulse energy. Constant pulse energy is to be understood as at least essentially the same pulse energy, e.g. granting a 10% tolerance for such restriction.
- The optical element may be configured for deflecting the laser pulses backscattered from the target towards the receiver.
- In the following, the invention will be described in detail by referring to exemplary embodiments that are accompanied by figures, in which:
- Fig. 1:
- shows an aircraft flying over a ground surface (target), a flight path (trajectory) of the aircraft, an airborne laser scanner according to the invention arranged on the aircraft, a point cloud of the swath of the aircraft generated by the airborne laser scanner, and laser pulses emitted by the airborne laser scanner;
- Fig. 2a,b:
- show two embodiments of the airborne laser scanner;
- Fig. 3a,b:
- show the point clouds of two exemplary laser pulse emission patterns according to prior art;
- Fig. 4a,b:
- show the point clouds of two exemplary laser pulse emission patterns according to the invention;
- Fig. 5a,b:
- show two further embodiments of the airborne laser scanner;
- Fig. 6a-d:
- show four graphs of embodiments of the pulse modulation according to the invention;
-
Figure 1 shows an aircraft 1 flying over a ground surface astarget 3, a flight path 6 (trajectory) of the aircraft 1, anairborne laser scanner 2 according to the invention arranged on the aircraft 1,laser pulses 4 emitted by the airborne laser scanner 1, and apoint cloud 5 of the swath of the aircraft 1 generated based on said laser pulses. The laserpulse emission pattern 4 is symbolically shown as a pyramid. In reality however it rather appears as a cone or fan. -
Figure 2a symbolically shows an embodiment of anairborne laser scanner 2 according to the invention.Figure 2b symbolically shows a comparative example of anairborne laser scanner 2. -
Figure 2a shows theairborne laser scanner 2 comprising a computer 26, an emitter 21 and a receiver 25, which in this example are combined in one unit, but may however also be arranged separately. Aprism 22, in particular a wedge prism, as optical element is in operative connection with amotor 24, such that it is rotatable around the rotation axis R. The movement of theoptical element 22 induced by the motor may be a continuous or a partial rotatory movement. A continuous rotatory movement is to be understood as at least one full 360°-rotation, in particular a permanent sequence of full rotations, and a partial rotatory movement is to be understood as a rotation, in particular a rotation of less than 360°, which is reversed in a defined scheme. For example, after a 180°-rotation, the motor changes the rotational direction and goes back the 180°, and so on. - During the movement of the
optical element 22, the emitter 21 emits a plurality of consecutive laser pulses 211 towards the target. The pulse path is altered by the movement of the optical element. The pulses 251 backscattered from the target are received by the receiver 25. - The computer 26 is connected to the emitter 21, the receiver 25, and the
motor 24, and it is configured for controlling these components. - In an embodiment, the computer 26 may additionally be configured for determining a distance from the airborne laser scanner to the ground surface for emitted and received laser pulses based on the time of flight method. Since the angle by which the optical element is deflecting the pulses and the direction of the pulse path are known and/or determinable (e.g. by an angle encoder), the TOF-distance value can be associated to the direction (e.g. at least one coordinate such as angle(s)) of the current pulse path at a specific measurement time.
- Particularly, a three-dimensional point cloud based on these associations (point measurements) can be generated by an external computer in a post-processing. In this case, the internal computer 26 is merely configured to collect the data. The data may comprise time stamps of pulse transmission and pulse reception or distance values already calculated by means of said stamps, and transmission/reception direction.
- Alternatively, the computer 26 can be configured for generating said point cloud, in particular in real-time. According to the invention, the laser pulses i.e. the laser pulse rate, or the laser pulse spacings are modulated based on the directional component of the current pulse path in a horizontal direction perpendicular to a direction of the flight path.
- The invention allows generating a point cloud which has a more steady (or: even) point distribution and which has less point cluster. A comparative example of the
airborne laser scanner 2 is shown infigure 2b , wherein the construction is similar to the one shown infigure 2a but the optical element is amirror 23 instead of a prism. Also, themotor 24 does not perform full rotations as positioning, but performs oscillations around an oscillation axis O. In particular, the oscillation axis runs along (or: parallel to) the surface of the mirror. The oscillation axis can however also run elsewhere. In particular, the oscillation can also be understood as a partial rotatory movement. - By the oscillating positioning of the
mirror 23, the emitted laser pulses 211 are deflected towards the target and back along a pulse path. Said pulse path pivots laterally to the flight path. -
Figures 3a and 3b show the unfavourable point distribution inpoint clouds 5 of two exemplary laser pulse emission patterns according to prior art. The paper plane is theground surface 3. Thesepoint clouds 5 have been generated by laser pulses which have been emitted at a constant pulse rate or constant pulse spacing. It can be recognised that the point density in the area of areverse position 7 of the motor is relatively high compared to the area between the reverse positions 7. At thisreverse position 7 the directional component of the pulse path in a horizontal direction perpendicular to a direction of the flight path is maximal. - The pattern of
figure 3a corresponds to a laser scanner offigure 2a , wherein continuous full rotations are performed while the aircraft is moving along aflight path 6. - The pattern of
figure 3b corresponds to a laser scanner offigure 2b , wherein the motor oscillates the mirror within the reverse positions 7 while the aircraft is moving along aflight path 6. - The reverse position may in some embodiments also be defined as a position of the motor in which a laser pulse is deflected by the optical element at the largest angle with respect to a plumb-line of the aircraft and in a plane perpendicular to a flight path of the aircraft. In other words, in the reverse position of the motor, an emitted laser pulse measures the fringe of the swath of the scanner, i.e. the most lateral areas with respect to the trajectory of the aircraft. At this reverse position, the motor reverses the deflection to the respective other direction (towards the other edge of the swath) with respect to an axis perpendicular to the
flight path 6 infigures 3a and 3b . -
Figures 4a and 4b show the result of pulse modulation.Figure 4a is according to the invention. - The larger the direction component of the pulse path in a horizontal direction perpendicular to a direction of the
flight path 6, the larger the pulse spacing is. In other words, the closer the points of thecloud 5 are to thereverse lines 7, the lower the pulse rate is. Accordingly, a more even point distribution is achieved. - In further embodiments, an airborne scanner according to the invention can achieve various scan patterns.
Figure 5a shows a double prism arrangement configured to deflect the laser path twice - once by theprism 22 and once by the prism 22'. Both prisms may be motorised bymotor 24 and motor 24' respectively. In particular, said motors are configured for rotating the prisms in opposite directions. -
Figure 5b shows a double optical element arrangement configured to deflect the laser path twice - once by themirror 23 and once by theprism 22. Both themirror 23 and theprism 22 may be motorised bymotors 24 and 24'. The oscillation of themirror 23 may be superimposed by the rotation of theprism 22. - Possible scan patterns resulting from the arrangements shown in
figures 5a and 5b (or other combinations of mirrors and/or prisms) may have the shape of a flower or a spiral following a zigzag path. By varying the respective rotation/oscillation speeds and/or directions, various different patterns are achievable. - Be the pattern as it may, according to the invention, the pulse rate is decreased the more the pulses approach the
reverse lines 7, or in other words, the more the pulse paths are directed towards the edges of the swath. -
Figures 6a, 6b, 6c, and 6d each show exemplary pulse modulations provided by the computer to the emitter. The modulations all have in common that they are depending on the pulse path direction. - If the lateral (relative to the flight direction) component of the pulse path direction reaches a maximum, i.e. the laser pulses arrive at a
reverse position 7, thepulse spacing 8 reaches its maximum (or respectively: the pulse rate has a minimum). - Accordingly, if the motor reaches a position so as to send a laser pulse at the smallest angle, in particular 0°, with respect to a plumb-line of the aircraft and in a plane perpendicular to a flight path of the aircraft, the
pulse spacing 8 reaches its minimum (or respectively: the pulse rate reaches its maximum). In this case, the laser pulse is directed essentially onto theflight path 6 on the ground surface, i.e. said directional component of the pulse paths in a horizontal direction perpendicular to a direction of the flight path is minimal, in particular zero. - The pulse spacing may be gradually varied according to a sinusoidal characteristic (
figure 6a ), a linear zig-zag characteristic (figure 6b ), a wave characteristic (figure 6c ), or a saw tooth characteristic (figure 6d ). In another embodiment (not shown), the pulse spacing is varied according to a step characteristic, wherein the pulse spacing is kept constant for a step period and then jumps up step by step. In particular, any combination of the above mentioned pulse sequences may be applied. - The sequences of
pulses 4 indicated by the arrows infigures 6a-d are qualitative illustrations. In fact, the sequences ofpulses 4 differ slightly based on the according pulse spacing characteristic 8. Said courses ofpulse spacings 8 are also to be understood as qualitative illustrations, which may appear exaggerated. However, thegraphs 8 are intended to show that there may be various optional mathematical principles behind the way the pulses are modulated according to the invention. - Although the invention is illustrated above, partly with reference to some preferred embodiments, it must be understood that numerous modifications and combinations of different features of the embodiments can be made. All of these modifications lie within the scope of the appended claims.
Claims (11)
- An airborne laser scanner (2) configured to be arranged on an aircraft (1) for surveying a ground surface (3) along a flight path (6) of the aircraft, wherein the airborne laser scanner comprisesan emitter (21) configured for emitting a plurality of consecutive laser pulses (211) towards the ground surface,at least one optical element (22, 22', 23) configured for deflecting the laser pulses along pulse paths towards the ground surface, wherein the optical element is a prism,a motor (24, 24') configured for altering the pulse paths by moving the optical element, wherein the motor is configured for rotating the optical element around a rotation axis (R), resulting in a cone-shaped laser pulse emission pattern,a receiver (25) configured for receiving the laser pulses backscattered from the ground surface, anda computer (26) configured forwherein surveying the ground surface is for generating a three-dimensional point cloud based on the time stamps and on the directions of the pulse paths, characterised in that the computer is configured for triggering the emitter to emit the laser pulses with a varying pulse spacing based on the directional components of the pulse paths in a horizontal direction perpendicular to a direction of the flight path.controlling the emitter, the motor, and the receiver,collecting time stamps of the emitted laser pulses and the received laser pulses or collecting distance values calculated based on time stamps of the emitted laser pulses and the received laser pulses, anddetermining directions of the pulse paths,
- The airborne laser scanner (2) according to claim 1,wherein the pulse spacing is gradually varied between a minimum pulse spacing and a maximum pulse spacing, whereinthe minimum pulse spacing is set when the directional component is minimal, andthe maximum pulse spacing is set when the directional component is maximal.
- The airborne laser scanner (2) according to any of the preceding claims, wherein the pulse spacing is gradually varied (8) according to a sinusoidal characteristic, a linear zig-zag characteristic, a wave characteristic, a saw tooth characteristic, a step characteristic, or any combination of said characteristics.
- The airborne laser scanner (2) according to any of the preceding claims, comprising an angle encoder configured for providing positions of the optical element.
- The airborne laser scanner (2) according to any of the preceding claims, wherein the computer (26) is configured for calculating a time-based sequence 84) of the varying pulse spacings.
- The airborne laser scanner (2) according to any of the preceding claims, wherein the optical element (22, 22') is arranged relative to the emitter (21) in such a way that the optical element deflects the laser pulses (211, 251) in a defined constant angle relative to the rotation axis (R) or relative to the oscillation axis (O).
- The airborne laser scanner (2) according to any of the preceding claims, wherein the computer (26) is configured fordetermining a current of the motor (24, 24'), anddetermining the directions of the pulse paths (211, 251) based on the current.
- The airborne laser scanner (2) according to any of the preceding claims, wherein the computer (26) is configured for receiving flight data from the aircraft (1), said flight data comprising a direction of the flight path (6) of the aircraft.
- The airborne laser scanner (2) according to any of the preceding claims, comprising an Inertial Measuring Unit (IMU), wherein the computer (26) is configured for receiving heading data from the IMU and for determining a direction of the flight path (6) based on said heading data.
- The airborne laser scanner (2) according to any of the preceding claims, wherein the computer (26) and the emitter (21) are configured for providing the laser pulses (211) with constant pulse energy.
- The airborne laser scanner (2) according to any of the preceding claims, wherein the optical element (22, 22', 23) is configured for deflecting the laser pulses (251) backscattered from the ground surface (3) towards the receiver (25).
Priority Applications (4)
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DK18153160.9T DK3517998T3 (en) | 2018-01-24 | 2018-01-24 | AIRBORNE LIDAR PULSE FREQUENCY MODULATION |
EP18153160.9A EP3517998B1 (en) | 2018-01-24 | 2018-01-24 | Airborne lidar pulse rate modulation |
CN201910018778.8A CN110068807B (en) | 2018-01-24 | 2019-01-09 | Pulse rate modulation for airborne lidar |
US16/255,502 US11639987B2 (en) | 2018-01-24 | 2019-01-23 | Airborne lidar pulse rate modulation |
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EP18153160.9A EP3517998B1 (en) | 2018-01-24 | 2018-01-24 | Airborne lidar pulse rate modulation |
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EP3517998B1 true EP3517998B1 (en) | 2023-12-06 |
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EP (1) | EP3517998B1 (en) |
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JP7313998B2 (en) * | 2019-09-18 | 2023-07-25 | 株式会社トプコン | Survey data processing device, survey data processing method and program for survey data processing |
US11614521B2 (en) * | 2021-04-21 | 2023-03-28 | Innovusion, Inc. | LiDAR scanner with pivot prism and mirror |
US20230078949A1 (en) | 2021-09-10 | 2023-03-16 | Leica Geosystems Ag | Airborne laser scanner |
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JP2008203123A (en) * | 2007-02-21 | 2008-09-04 | Japan Aerospace Exploration Agency | Water surface and ground surface observation device for aircraft |
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JP2008203123A (en) * | 2007-02-21 | 2008-09-04 | Japan Aerospace Exploration Agency | Water surface and ground surface observation device for aircraft |
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US20190227149A1 (en) | 2019-07-25 |
CN110068807A (en) | 2019-07-30 |
EP3517998A1 (en) | 2019-07-31 |
CN110068807B (en) | 2023-06-09 |
US11639987B2 (en) | 2023-05-02 |
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